1. Field of the Invention
The invention relates generally to circuitry design, more specifically, the invention relates to modeling of power distribution systems for a central processing unit system.
2. Background Art
As today's computer systems operate at frequencies exceeding 1 GHz, the demands on internal power supplies also increase. For instance, as the technology is scaled smaller and faster, the power supply voltage must decrease. However, as the internal clock rates rise and more functions are integrated into microprocessors and application specific integrated circuits (ASICs), the total power consumed must increase. These demands require the internal power supply to respond quickly and reliably without significant overshoot, undershoot, or ringing of the supplied voltage.
Obviously, the design of the power system is critical to meeting these stringent requirements. A critical part of the design process is the modeling of the system. Typically, a model is used to simulate the system's performance so that design decisions can be made based on its results. The key questions in developing a model are: (1) the level of complexity it will entail; and (2) the degree of accuracy it will provide with its results. As a general rule, a more complex model has greater accuracy in its results. However, a complex model may take several days of operation just to simulate a few micro-seconds of system time.
FIG. 1 shows a prior art depiction of a central processing unit (CPU) power distribution system 10 with power system components that must be simulated by such a model. The main circuit board 12 itself is the central platform with the system power supply board 14 and system ground board 16 layered underneath. Attached to the surface of the board 10 is the circuit package 18 that holds the central processing unit 20 or “chip”. Also shown are various components of the power system including: high-capacity ceramic capacitors 22; an air-core inductor 24; a regulating integrated circuit 26; switching transistors 28; a mid-capacity tantalum capacitor 30; and low-capacity electrolytic capacitors 32.
Of these components, the model of the chip 20 is the most difficult to develop. The components on the chip that must be modeled include the current draw of the chip as well as its intrinsic capacitance. The current draw generally includes characteristics such as average, maximum, and minimum currents at different processes and speed grades. Additionally, the chip model should allow multiple different current spikes at different known magnitudes and frequencies.
Modeling the current draw of the chip is accomplished by several methods including the use of transistors, resistors, or current sources. A transistor based model requires a tremendous amount of transistors to model the current performance over time. Additionally, the different transient currents must be included in the model. Also, the parasitic capacitance must be sized to be close to the actual value on the chip. The result is a complex circuit that does not scale well and has difficulty maintaining the proper amount of intrinsic capacitance. Finally, the circuit is so complicated that it has a simulation time that unacceptably long.
A resistor based model uses resistance controlled voltage to model the current performance over time. It is relatively easy to determine the necessary voltage and resistance that causes the average, maximum, and minimum currents. The intrinsic capacitance is easily modeled as a voltage controlled capacitor. However, the current draw during transient sweeps is hard to control when attempting to overlay different frequency and magnitude spikes. An additional problem involves modeling transient currents. The voltage controlled resistors often cause the transient currents to have a higher than accurate frequency. The result is that while a resistor based model has a short simulation time, it is not accurate in certain circumstances.
A current source based model uses explicit current sources to model the current performance over time. A voltage controlled capacitor is used to model the intrinsic capacitance. This model has an even faster simulation time than the resistor based model. However, the current sources cannot be used in the AC sweeps since they are not represented as resistors in the AC domain. As with the resistor based model, the current source based model has excellent simulation time but it is not accurate in certain circumstances.